The science behind the numbers

Your results are a starting point—not the whole story.

Around one in six people worldwide experience infertility during their lifetime, and the NHS estimates that around one in seven couples in the UK may have difficulty conceiving. [1,2,16]

Male factors contribute in about half of couples with infertility. Yet the man can still receive little more than a semen result and a brief instruction to improve his lifestyle. [1,2,16]

Population figures show why male fertility deserves attention. They cannot tell an individual man what is happening in his body, predict whether he will become a father or replace a clinical assessment.

Start your assessment See our evidence sources

Unexplained should not mean under-investigated

Infertility may be described as unexplained when standard investigations have not identified a cause. [6,7,16]

genuinely unexplained after appropriate investigation

simply not investigated far enough to answer the clinical question

Estimates vary because studies and services do not all investigate in the same way. Current European guidance notes up to 30% of couples remain in this category, and that in up to 50% of men with impaired semen quality, no specific cause is identified. [6,7,16]

Sometimes uncertainty remains after careful assessment. We cannot promise otherwise. The important distinction is between genuinely unexplained after appropriate investigation and simply not investigated far enough to answer the clinical question, something we sadly hear far too often.

A semen analysis tells us what. We also want to understand why.

A semen analysis describes features of the sample, including sperm concentration, total count, movement and shape. [16]

These measures matter, but they vary between samples and do not create a clean line between fertile and infertile men. Results need to be considered alongside the fertility history, the other partner's circumstances and relevant clinical findings. [16]

Depending on the question, useful context may include hormones, testicular size and structure, varicocele, childhood or medical history, previous surgery or infection, medicines, sexual and ejaculatory function, genetics, general health and selected sperm-function tests. Not every man needs every investigation, but every man needs a personalised plan.

Films · in production

Teaching extract · Footage · 35–60 s · In productionA GP’s Guide to Male Fertility, or the Male Fertility PodcastRights to be cleared

Film 6 · Animation · 60 sWhat a semen analysis actually measures

Film 7 · Animation · 77 sWhere male infertility can arise

Film 6

What a semen analysis actually measures

A semen analysis is a snapshot and the values must be interpreted together.

Sample 1Sample 2WHO reference limits

Volume

Concentration

Total number

Progressive motility

Non-progressive motility

Morphology

Vitality

1 / 9One sample

2 / 9Volume

3 / 9Concentration

4 / 9Total number

5 / 9Progressive motility

6 / 9Non-progressive motility

7 / 9MorphologyFour per cent normal morphology does not mean that 96% of sperm are useless. Morphology uses exceptionally strict laboratory criteria.

8 / 9Vitality

9 / 9Two samples from the same man

WHO reference limits are not a pass–fail boundary between “fertile” and “infertile”. The EAU specifically notes that semen analysis alone cannot make that distinction. EAU guideline

Words awaited from Anand

Film 7

Where male infertility can arise

The same semen result can have several different causes.

GnRH
LH
FSH
Testosterone

1 / 5Factory

2 / 5Hormonal signal

3 / 5Plumbing

4 / 5Delivery

5 / 5Sperm function/DNA

impaired testicular production

varicocele

previous undescended testes

hormonal suppression

obstruction

infection

ejaculation problems

DNA damage

An abnormal result describes what was found; it does not explain why it happened.

Words awaited from Anand

Semen health can be part of a wider health conversation

Research has found associations between impaired semen quality and some later health outcomes. [15]

Those findings are important enough to take male reproductive health seriously, but an association is not proof that one causes the other. A semen result is not a longevity test and cannot predict one person's lifespan. [15]

For an individual man, an unexpected semen result may be a sensible prompt to review symptoms, hormones, metabolic health, medicines and lifestyle with a clinician. It is information to interpret, not a diagnosis of hidden disease.

Sexual function belongs in fertility care

Fertility difficulty and treatment can affect desire, erections, ejaculation, enjoyment and sexual self-confidence. [17,18]

Scheduled intercourse and repeated disappointment can place strain on both partners. These effects are documented in clinical guidance and should be asked about without shame or assumptions. [17,18]

A sexual difficulty can also make conception harder in a very practical way. It deserves its own assessment rather than being dismissed as stress or described as a performance failure.

Are sperm counts falling?

A large 2023 meta-analysis estimated substantial falls in average sperm concentration and total sperm count (62.3%) in men not selected for fertility between 1973 and 2018. [8,14]

Other studies have found different patterns in particular populations. These are observational trends assembled across countries, decades and laboratory methods; they do not identify one cause and they cannot tell a man how his own fertility has changed. [8,14]

The responsible conclusion is not that every man's fertility is collapsing. It is that male reproductive health deserves good measurement, continued research and more serious clinical attention.

What about testosterone?

Researchers have also reported changes in average testosterone levels across time. [9]

A 2026 conference abstract described downward trends across several datasets, but the evidence has not yet matured into a settled percentage that should be applied to the public or to an individual patient. We will update this page when a full peer-reviewed analysis is available. [9]

For England and Wales, the total fertility rate fell from 1.94 children per woman in 2010 to 1.39 in 2025. [10,11]

Fewer births are not the same as more infertility

In 2025, the total fertility rate in England and Wales reached 1.39 children per woman, and the number of live births was the lowest since 1977. [10–12]

That is a demographic measure, influenced by when and whether people choose or are able to have children, as well as social and economic conditions. It is not a measurement of biological infertility and should not be presented as one. [10–12]

What this means for one man

Do not treat a single result as a verdict.

Repeat or extend testing only when it answers a defined question.
Include sexual function and general health in the conversation.
Consider both partners' circumstances and the time available.
Ask how certain a finding is and whether it would change a decision.

Access without understanding isn’t progress

The honest promise

We cannot find a cause in every case, and more testing is not automatically better. We can provide a purposeful investigation, careful interpretation and a clear explanation of what is known, what remains uncertain and what could usefully happen next.

Start your assessment Explore Fertility Made Simple

Evidence sources

[1] World Health Organization. ‘1 in 6 people globally affected by infertility.’ 4 April 2023. WHO source
[2] NHS. ‘Infertility: overview.’ Accessed 9 September 2026. NHS source
[3] European Society of Human Reproduction and Embryology (ESHRE). Impacting infertility: a research agenda. March 2025. ESHRE report
[4] Eunice Kennedy Shriver National Institute of Child Health and Human Development. ‘How common is male infertility, and what are its causes?’ Last reviewed 30 July 2021. NICHD source
[5] National Institute for Health and Care Excellence. Fertility problems: briefing paper / guideline context (historical UK cause estimates). NICE source
[6] Practice Committee of the American Society for Reproductive Medicine. Evidence-based treatments for couples with unexplained infertility: a guideline. Fertility and Sterility. 2020;113:305–322. ASRM guideline
[7] Romualdi D, et al. Evidence-based guideline: unexplained infertility. Human Reproduction. 2023;38(10):1881–1890. doi:10.1093/humrep/dead150. DOI
[8] Levine H, et al. Temporal trends in sperm count: a systematic review and meta-regression analysis of samples collected globally in the 20th and 21st centuries. Human Reproduction Update. 2023;29(2):157–176. doi:10.1093/humupd/dmac035. DOI
[9] Levy A, et al. L26/O-204 Temporal trends in total and free testosterone (1972–2019): a systematic review and meta-trend analysis. Human Reproduction. 2026;41(Suppl 1):deag083.202. Conference abstract. doi:10.1093/humrep/deag083.202. DOI
[10] Office for National Statistics. Births in England and Wales: 2025 (final rates). 8 September 2026. ONS bulletin
[11] Office for National Statistics. ‘How is the fertility rate changing in England and Wales?’ 28 October 2024. ONS explainer
[12] University of Southampton, Centre for Population Change. ‘Understanding and Projecting Childbearing Trends in the UK.’ 26 September 2025. Research summary
[13] Quaas A, Dokras A. Diagnosis and treatment of unexplained infertility. Reviews in Obstetrics and Gynecology. 2008;1(2):69–76. Full text
[14] Priskorn L, Nordkap L, Bang AK, et al. Average sperm count remains unchanged despite reduction in maternal smoking: results from a large cross-sectional study with annual investigations over 21 years. Human Reproduction. 2018;33(6):998–1008. doi:10.1093/humrep/dey090. DOI
[15] Priskorn L, Lindahl-Jacobsen R, Jensen TK, et al. Semen quality and lifespan: a study of 78,284 men followed for up to 50 years. Human Reproduction. 2025;40(4):730–738. doi:10.1093/humrep/deaf023. DOI
[16] European Association of Urology. EAU Guidelines on Sexual and Reproductive Health: Male Infertility. Limited update March 2026. EAU guideline
[17] Practice Committee of the American Society for Reproductive Medicine. Diagnostic evaluation of sexual dysfunction in the male partner in the setting of infertility: a committee opinion. Fertility and Sterility. 2023;120:967–972. ASRM committee opinion
[18] Gameiro S, Boivin J, Dancet E, et al. ESHRE guideline: routine psychosocial care in infertility and medically assisted reproduction. Human Reproduction. 2015;30(11):2476–2485. doi:10.1093/humrep/dev177. DOI